Bromobenzene is widely used as an additive in motor oils and as a heavy liquid solvent especially where mass crystallization is required. Further, it is used as a starting material in several organic syntheses, especially in the preparation of organometallic reagents such as phenyl magnesium bromide, phenyl lithium and diphenyl zinc which in turn are employed in the preparative synthesis involving carbon-carbon bond formations.
Reference may be made to PCT-international application No 88,07,513 to M. Rule et al. wherein an improved vapor phase bromination of aromatic compounds was disclosed. The reaction was carried out in the presence of oxygen and an iron containing silica-alumina catalyst at 351° C. Hydrobromic acid was used as a bromine source. The conversion of aromatic compound (substrate) to the desired bromo compound was only 60 percent. The drawbacks of this method are that the experimental temperature is too high and the brominating agent via hydrobromic acid is highly corrosive even at room temperatures. Moreover, this method requires a catalyst and the yields are only 60%. Conducting the vapor phase reaction at high temperatures particularly with hydrocarbons in the presence of oxygen is highly hazardous and risky.
The paper entitled Molecular sieves as catalyst for aromatic bromination in Zeolite 1987, 7(6), 499 by J. Zabicky, et al discloses the ring bromination of aromatic substrates using liquid bromine as brominating agent and in the presence of mordenite and 13X type zeolite containing metallic iron under reflux temperature. Quantitative yields of p- and o-bromoderivatives were obtained when the reaction was carried out in the absence of sunlight. The main limitations of this method are that the reaction should be carried out in sun light to avoid the formation of dibromo derivatives. Hazardous liquid bromine is the brominating agent which requires special handling precautions and equipment. The requirement of mordenite and 13X type zeolite containing metallic iron is essential for ensuring the product formation. Moreover the bromination of benzene is incomplete even at reflux temperature when sun light is absent. The Japanese patent, JP 6293,242 to H. Ishida et al discloses the preparation of p dibromobenzene by oxidative bromination of benzene using liquid bromine and/or hydrobromic acid as brominating agents. The reaction was carried out in a gaseous phase at 200° C. in presence of oxygen. In this reaction the rate of reaction was increased by the use of sodium zeolite containing 18 percent copper. The draw backs of this method are that the reaction should be conducted in gaseous phase at 200° C. in the presence of oxygen. Sodium supported zeolite containing 18% copper ions is required to catalyze the reaction. It gives 95% dibromobenzene and negligible quantity of bromobenzene with 45% conversion. Moreover, the brominating reagents used here are very corrosive, toxic and need special skills and equipment to handle them.
A paper entitled “Oxidative bromination of aromatic compounds catalysed by the hetero poly acids” in Kinet. Catal. 1982, 23(4), 992 by T. V. Gorodeestsaya, describes the use of a hetero poly acids containing molybdenum and vanadium to catalyze the reaction by hydrobromic acid and oxygen in aqueous acetic acid. The hetero poly-acid catalyzes the oxidation of bromide ions by oxygen to form bromine which was further used in the ring substitution reactions of aromatic compounds. The drawbacks of this method are that the experimental temperature is too high and the brominating agent, hydrobromic acid is highly corrosive even at room temperatures. Moreover, this method requires a catalyst and the yields are only 60%. Conducting the vapor phase reaction at high temperatures particularly hydrocarbons in the presence of oxygen is highly hazardous and risky.
The Japanese patent JP 7,762,201 to Y. Toteraishi, et al discloses the process of bromination of organic compounds using liquid bromine. The byproduct hydrobromic acid produced in the reaction was oxidized using chlorine gas which was further used up to complete the bromination reaction. The drawbacks of this process are that it requires highly toxic and corrosive liquid bromine as brominating and chlorine as oxidizing agents which require special apparatus and operation skills.
The patent, JP 7616,620 to T. Asai et al discloses the preparation of hexabromobenzene having high melting point. In this process benzene was directly added to mixture of liquid bromine and anhydrous aluminium chloride maintained at 40° C. and the entire mixture stirred for four hours. Water was added and the temperature was raised to 70° C. to remove bromine. Finally the pH of the solution was adjusted to 9 with sodium carbonate solution to obtain 97% hexabromobenzene having melting point 325° C. In this method it was possible to obtain only hexabromobenzene and not bromobenzene. The draw back of this method is that it involves a neutralization step and precautionary measures to handle corrosive and hazardous liquid bromine. Further, this method requires anhydrous aluminium chloride as catalyst which unnecessarily increases process steps and thus effect the production cost.
The Japanese patent JP 7420,126 to T. Komiyama, et al discloses the bromination of aromatic hydrocarbons containing either halo or carboxyl group by treatment with liquid bromine in the presence of aqueous solution of ferric bromide under 5 kg/cm2 nitrogen pressure. Then, the pressure was raised to 30 kg/cm2 with oxygen and the reaction mixture was heated to 200° C. for one hour. The drawback of this method are that it employs liquid bromine, which is hazardous and corrosive. Beside the reaction is carried out at a very high pressure and temperature which is cost effective.
In Japanese patent JP 7436,633, to T. Komiyama, et al disclose the preparation of bromobenzene by the reaction of benzene with ammonium bromide in the presence of support material containing copper bromide. In this process, benzene-ammonium bromide-steam-air in definite volume percent was passed through the catalyst at 300° C. with contact time of 2.5 seconds. The drawbacks of this method are that it is a vapor phase reaction and reaction temperature is too high. Moreover, conducting this vapor phase reaction at high temperatures particularly with hydrocarbons in the presence of oxygen is highly hazardous and risky. Maintaining the steam to air in definite volume percent is also difficult.
O. A. Sadygor et al in USSR patent SU 1,468,896 have disclosed the preparation of bromobenzene in high yield. In this process, the bromination of benzene was carried out at 20–40° C. with 1–1.3 mole equiv of sodium bromide or potassium bromide in the presence of 1–1.3 mole equiv of sodium hypochlorite in an acidic medium containing either 15–36.5% aqueous hydrochloric acid or 15–96% aqueous sulphuric acid. The reaction mixture was aged for 3–5 hours at 20–40° C. The main limitation of this method is that the maximum concentration of hypochlorite solution available is 4%, as a result large containers are required to handle large volumes hypochlorite solutions in large scale production of bromobenzene.